1. Field of the Invention
The invention relates to a method and an apparatus for power factor correction for a non-ideal load that is supplied from a mains power supply by a compensation device. The compensation device is electrically connected in parallel with the load and has a pulse converter with at least one capacitive store, a matching filter and a regulating and control device.
A method and an apparatus for carrying out this method are known from the publication "Optimal Control and Appropriate Pulse Width Modulation for a Three-Phase Voltage dc-Link PWM Converter" by R. Marschalko and M. Weinhold, printed in the IEEE-IAS Conference Proceedings of the IAS Annual Meeting in Houston/Tex., October 1992.
The increasing use of non-linear loads (in particular diode rectifiers as are used, for example, in power supply units for PCs and televisions) in mains power supply systems is increasingly distorting the mains power supply voltage. Specifically, their currents have high harmonic levels and cause voltage drops across the mains power supply impedances which are superimposed on the originally sinusoidal mains power supply voltage. At excessively high levels, the voltage distortion can lead to overloading of the mains power supply equipment (for example transformers, compensation systems) and can interfere with correct operation of other loads.
Public electricity suppliers and international working groups have thus issued recommendations relating to the maximum permissible voltage distortion which a load may cause. So-called compatibility levels have been defined for individual harmonics in low-voltage mains power supplies. Equipment manufacturers must develop their products such that they still function without any interference at these distortion levels. The public electricity suppliers have to ensure that the compatibility levels in their mains power supply systems are not exceeded. However, the mains power supply voltage distortion in many mains power supply systems has already reached the compatibility level, and a further increase is expected.
A further problem in distribution networks is the VAr requirement of, for example, mains-commutated thyristor converters or asynchronous machines in industry, which depends on its operating point and thus in general varies. On the basis of contractual agreements with the public electricity supplier involved, the respective operator of such equipment often has to ensure that he maintains a specific power factor with respect to the point of common coupling to the higher-level network.
For example, single-phase modes cause an unbalanced load on the three-phase mains power supply. In terms of the mains power supply short-circuit rating, large loads cause load currents which result in large unbalanced mains power supply voltage drops across the mains power supply impedances. These can interfere with correct operation of the loads.
Large, short-term real power requirements from loads can result in an increased energy tariff price since the public electricity supplier involved makes a portion of the energy costs dependent on the maximum real power drawn during a given time interval (frequently one year). By using an energy store, which emits previously stored energy during this peak load time, the power peak can be capped, and the power price can be reduced. The use of energy stores can also avoid expensive mains power supply upgrading.
Until now, the problem of load current harmonics and the mains power supply voltage distortion caused by them has been solved by conventional filter circuits. Since the mid 1980's, active filters have also been used, based on control methods in both the time domain and the frequency domain. Various active filters were introduced in the Conference Report entitled "New Trends in Active Filters" by H. Akagi, printed in the Conference Proceedings of EPE' 95 in Seville, pages 17 to 26.
Compensation for VAr, or power factor correction, is nowadays normally still carried out conventionally using regulated compensator banks, with or without inductors. For a number of years, solutions with mains-commutated converters, so-called Static Var Compensators (SVC) and with self-commutating GTO or IGBT converters, so-called Static Condensers (STATCON) have also been used.
Such a compensation device with an IGBT pulse converter has been described in detail in the publication entitled "Development of FACTS for Distribution Systems" by D. Povh and M. Weinhold, printed in the Conference Proceedings of the EPRI Conference on the Future of Power Delivery, April 9-11, 1996. Such a compensation device is also called a Power Conditioner, in particular a Siemens Power Conditioner (SIPCON). Such a power conditioner has a pulse converter that is coupled to the mains power supply in parallel and via an LCL filter. The object of the LCL filter is to reduce the switching frequency feedback from the drive unit for pulse-width modulation. The converter used, which was developed for regulated-speed drives and has been manufactured in large quantities with ratings in the range from 2 kVA to 1.5 MVA, represents the basis of the SIPCON. The pulse converter contains insulated gate bipolar transistors (IGBT) and operates at switching frequencies up to 16 kHz. The power conditioner can be expanded by an energy store in order to bridge drops in the real power and to compensate for load fluctuations. The normal application is parallel coupling. This type of coupling is the most suitable for controlling voltage fluctuations resulting from VAr and for filtering low-order harmonics from a load. A further possibility is to connect the power conditioner in series. This application is advantageous if the load is intended to be supplied with an improved voltage quality or if transient mains power supply voltage fluctuations occur frequently.
The power conditioner control structure is configured such that it is possible to switch between three modes. On the input side, the regulator has a space vector transformation device with which a mains power supply current space vector, a conductor voltage space vector and a compensator current space vector are generated from measured mains power supply currents, conductor voltages and compensator currents. The space vectors are digitized and fed to a voltage regulator, power factor correction and a flicker regulator, it being possible to supply the outputs from these regulators via a changeover switch to a pulse-width modulator in the pulse converter.
If the voltage regulation mode is chosen, the mains power supply voltage space vector is compared with a reference value. A PI regulator then determines the VAr that is required to eliminate the voltage error. The output value from the VAr regulator is passed to the pulse-width modulator.
Load balancing has until now normally been carried out using Steinmetz circuits. These consist of reactive elements (capacitors and conductors) which are connected as required via switches or converters (for example three-phase controls).
Until now, energy stores have mainly been used as spare capacity for power station failures and frequency regulation. The Published, Non-Prosecuted German Patent Application No. 42 15 550 discloses a device for providing electrical energy from a DC store for an AC network, a superconductive magnetic energy store (SMES) with a very high storage efficiency being used as the DC store.
In the Conference Report (Houston) mentioned above, control methods are introduced so that the power conditioner can compensate for a fundamental shift VAr. In order to keep undesirable load reactive current elements away from the mains power supply, the compensation device must feed these elements in parallel to the load so that the current elements of the compensator at the point of common coupling (PCC) cancel out the load reactive current elements. The reactive current elements contained in the mains power supply current are first of all calculated from the mains power supply voltage and current space vectors, for this purpose. The difference between the compensator voltage space vector on the mains current side and the mains power supply voltage space vector must now be used to produce, via the coupling filter which is represented as a compensator inductance, a current space vector which keeps the undesirable reactive current elements away from the mains power supply and, in addition, supplies the DC circuit. The object of regulating the introduced power conditioner is to determine the transfer function space vector (required to produce this voltage) between the intermediate circuit voltage of the pulse converter and the compensator voltage space vector on the mains power supply side.
The determined mains power supply voltage space vector and the determined complex-conjugate mains power supply current space vector are used to calculate the instantaneous VAr, which is supplied to a PI regulator whose output gives an angle value indicating the shift in angle between the mains power supply voltage space vector and the transfer function space vector. The angle and a unit space vector which points in the direction of the transfer function space vector as well as a constant amount are used to generate the transfer function space vector, which is supplied to the pulse-width modulator of the pulse converter. The pulse converter produces a compensator voltage space vector at its output on the mains power supply side as a function of the voltage across the capacitive store and of the transfer function space vector, and this compensator voltage space vector drives a compensation current via the inductance of its matching filter.
If the load now requires fundamental shift VAr, then this is initially drawn from the mains power supply. The occurrence of this VAr in the mains power supply changes, via the PI regulator, the angle between the mains power supply voltage space vector and the compensator voltage space vector on the mains power supply side. This leads to a compensator current space vector being formed that, inter alia, contains a real element. This leads to real power being interchanged between the mains power supply and the pulse converter intermediate circuit, and to a change in the intermediate circuit voltage. The angle and the intermediate circuit voltage now change until the fundamental shift VAr occurring in the mains power supply has disappeared. In the steady state, the angle is then once again equal to zero and the compensation device supplies exactly that fundamental shift VAr that the load requires. However, the intermediate circuit voltage has changed from that on no-load.
If it is assumed that the load requires inductive fundamental shift VAr, then the compensation device must emit capacitive VAr and the magnitude of the compensator voltage space vector on the mains power supply side is greater than the mains power supply voltage space vector. In consequence, the intermediate circuit voltage rises from that on no-load, and is set as a function of the operating point.
An ideal, three-phase mains power supply provides the load with three purely sinusoidal voltages at a constant frequency, which are shifted through 120.degree. electrical with respect to one another and have constant, identical peak values. The ideal mains power supply currents for this network are proportional to the corresponding line-to-earth mains voltage in each phase, the proportionality factor being the same in all three phases. Specifically, a desired amount of energy or real power is then transmitted with the minimum collective root mean square current, and thus with the minimum possible load on the mains power supply. The currents are therefore called real currents. From the mains power supply point of view, such an ideal load acts like a three-phase, balanced, non-reactive resistor.
Every load whose behavior differs from this causes current elements which contribute nothing to real power transmission. These are called reactive currents. Subject to the precondition that the supply voltage corresponds approximately to the ideal case mentioned above, these reactive currents include the harmonic currents (including a DC element), whose frequencies are a multiple of the mains frequency, the fundamental shift reactive currents, which are caused by the phase shift between the mains power supply voltage fundamental and the mains power supply current fundamental, and the fundamental unbalance reactive currents, which are caused by unbalanced loads.